Last night, commuting, I got lots of dirty water all over my back brake (Promax Render-r mechanical disc), as well as on most of the rest of the bike, and me. I wiped the rotors with a clean rag to get the grit off, because light braking made an unpleasant grinding sound. This morning I was about 30km into a rather long ride when I realised I had essentially no back brake. I don't use the back much except to hold back on descents, though I do normally take up the slack when stopping, so it was the first decent descent when I discovered this.

I can only assume that the water was oily (certainly possible given the state of the roads and the deep puddles last night) and that I spread it evenly over the disc.

So I ended up riding 250km with essentially no back brake. I would have tried cleaning the rotor with an alcohol sterilising wipe from my first aid kit, but they'd dried up. I did get a bit off slowing (not stopping) power from holding the brake on all the way down long descents, and cooking it, but presumably this just moved the oil to the pads and finished them off.

I've ordered new pads (front and rear), which I needed soon anyway. I plan to degrease the rotor, put the used front pads in the back, and see what happens.

Now I've had the chance to remove the wheel and pads, the moving pad on the rear is unevenly worn -- much more wear towards the centre of the rotor. It was seated properly; I checked after takling the wheel off and before taking the pads out. I don't know whether this wear was cause or effect, as I had the back brake on hard on most downhills for the little effect it did have. There are some scratches on the back rotor but not much more than the front (or the MTB) and nothing deep like I'd expect if there was a bit of grit preventing proper contact. The pads were by no means clean but not gritty either.

But what could I have done at the side of the road? There were no bike shops around, but convenience stores and I could probably have found a pharmacy.

  • If my assumption about why I had no stopping power seems unlikely, please say so, and I'll ask a question.
    – Chris H
    Mar 10, 2018 at 21:20
  • I'm going to go add a wad of absorbent paper towels in a plastic baggie in my tool kit right now. Mar 10, 2018 at 21:59
  • Speaking from MTB experience, I would be very surprised if Contamination from puddle splashes causes that much drop in performance. Thoughts are its a combination of a bit of contamination, the grit wore the pads down, or have got into the brake cable.
    – mattnz
    Mar 11, 2018 at 4:26
  • @mattnz so am I. The movement seems good, and without having removed the pads completely I can see plenty of material there. So I couldn't think of anything else
    – Chris H
    Mar 11, 2018 at 7:06
  • 1
    "wiped the rotors with a clean rag" Maybe this is where you went wrong? Was braking significantly degraded before this wipe down, or just noisy? I've always heard, anecdotally, to never wipe a rotor with anything other than brand new paper towel with isopropyl alcohol or other braking-surface specific cleaner. How clean was the rag? If it's a shop rag that was rinsed under a tap or ran through a clothes washer, probably not clean enough (laundry detergent). I biked across Canada with mech disks, rode through nasty rainstorms on trucking routes, never had issues with brakes degrading.
    – SSilk
    Mar 14, 2018 at 18:36

4 Answers 4


Oil and contaminants from the road will float to the top of these puddles and get on the rotors.

Unfortunately once rotors are contaminated, it is EXTREMELY difficult to decontaminate them due to the porous nature of the steel they use for the discs. When you throw fresh pads on, the rotor can contaminate the pads again.

As far as brake pad decontamination, there is hope. What we do here is

  1. Heat the pad up with a blow-torch to expand the pad material
  2. Spray a degreaser on the pad until it stops evaporating instantly
  3. Sand the pad with 120 grit sandpaper until you see new pad material
  4. Clean the pad off by hitting it with the blow-torch or isoprop-alcohol
  5. Reinstall

As far as what could be done on the side of the road, you can try tightening the barrel adjuster for your brake until you can get some form of grip or wiping the contaminants off with the inside of your jersey (That's about it).

If you suddenly lose braking power to both wheels, you can try what BMX riders do and stick the heel of your shoe between the top of your rear tire and your frame.

  • 1
    That's interesting that you say the rotors are harder to degrease than the pads, I was expecting it to be the other way round. The BMXers trick probably wouldn't work here, as I've got full mudguards (refitted after the filthy wet commute).
    – Chris H
    Mar 11, 2018 at 7:03
  • If your mudguards are the flexible plastic and look vaguely like a motocross mudguard, then you can treat them like a modern spoon brake and simply press them into the wheel with your foot. Downside if the "leading edge" catches any tread it could be a sudden stop. Also you might wear clean through your mudguard, and then an exposed edge could catch.
    – Criggie
    Mar 11, 2018 at 9:16

I may never get to the bottom of this, and would still like to know what roadside repairs would be possible.

I do know that cleaning the rear rotors withs meths (denatured alcohol) and putting the old front pads on the back (complete with bedding in) got it back to a working condition. They're possibly not a sharp as the front brake with new (bedded in) pads, but it's plenty for the back -- I had to hang off the back of the saddle to keep enough grip on the road to bed them in properly. While the moving pad had become very worn (and with an angle on it) I reckon this was caused by leaning on the brakes on all the downhills to try to get something out of them. I did inspect the pads when the symptoms first appeared (without removing the wheel) and would have noticed a non-flat pad then, or anything between the pad and rotor (which is what I was looking for).

I don't particularly want to test this again but will carry some IPA wipes (as used before injections). While alcohol-based wipes are no longer recommended for cleaning wounds they're still handy to have in a first aid kit especially if your hands might be greasy when dressing road rash, so they'll do double duty.

Coming back to this after some time, the isopropanol wipes work well. For several days after heavy rain the puddles smell of diesel and have quite a sheen on them passing a farm near here, which gives me more opportunities to test this than I'd really like.

  • I'll accept this to close the question if there's nothing better.
    – Chris H
    Mar 21, 2018 at 22:06

For a 250 km (150 mile) ride, you should carry spares unless you're in a race.

But for a quick fix, pull the wheel out of the bike and run your trousers (?) belt through between the pads.

Riding with a buddy helps here, which is "another thing" to take on long rides.

  • 2
    You recommend to bring a spare rotor and spare brake pads on any longer ride? A torque limiting wrench as well? What does your second sentence mean? Remove the rear wheel and clean the pads with a belt? Why a belt?
    – gschenk
    Dec 16, 2018 at 10:55
  • @gschenk I'm guessing a leather belt will act something vaguely like a strop, cleaning contaminants from the brake pad's surface, enough to get you home. Not sure what you'd use if wearing cycling pants without a belt.... perhaps flax might be a useful substitute, or something made of nylon webbing.
    – Criggie
    Jun 21, 2020 at 12:33

I have exactly the same problem…recurrent. I’ve changed pads only to discover after a months use, the rear brake is next to useless at low speed. BUT I do have a road fix. You need a hill with at least 30m down hill. Attack it fast and use 3s bursts of rear brake repeatedly. This gets max energy and heat into the pads. As you continue to pump, you will notice brake squeal gives way to actual braking! I commute everyday and do this almost once a week- especially after rain. By the way, never had the problem on the front!

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